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Re: Real accuracy of the method of lunar distances
From: Frank Reed CT
Date: 2004 Jan 7, 04:50 EST
From: Frank Reed CT
Date: 2004 Jan 7, 04:50 EST
Regarding rates of change in apparent lunar distances, George Huxtable wrote:
"Yes, that can happen, particularly when the Moon is passing nearly overhead
in the tropics."
...passing nearly overhead ...and well away from the meridian. If the Moon is close to the meridian, the effect on a lunar from rate of change of parallax would not be serious even if the altitude is high.
And wrote:
"Measuring lunar distance to the Sun or a planet, rather than a star, slows the change of
lunar distance a bit further still."
But not always. It increases the rate of change if the Moon happens to be on the other side. The Sun chases a waxing moon.
And:
"Bill said that it was "due mostly to refraction and parallax", but in fact
it's almost entirely due to the effect of parallax."
In recommended cases, yes. Unless you get into lunars involving objects below 10 or 12 degrees altitude. At those altitudes and lower, the rate of change in the apparent lunar distance can be dramatically dominated by the rate of change in refraction. Those are also the altitudes where you need to worry about correcting for the flattening of the Sun/Moon (angular dependence for semi-diameter) when clearing the lunar distance.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"Yes, that can happen, particularly when the Moon is passing nearly overhead
in the tropics."
...passing nearly overhead ...and well away from the meridian. If the Moon is close to the meridian, the effect on a lunar from rate of change of parallax would not be serious even if the altitude is high.
And wrote:
"Measuring lunar distance to the Sun or a planet, rather than a star, slows the change of
lunar distance a bit further still."
But not always. It increases the rate of change if the Moon happens to be on the other side. The Sun chases a waxing moon.
And:
"Bill said that it was "due mostly to refraction and parallax", but in fact
it's almost entirely due to the effect of parallax."
In recommended cases, yes. Unless you get into lunars involving objects below 10 or 12 degrees altitude. At those altitudes and lower, the rate of change in the apparent lunar distance can be dramatically dominated by the rate of change in refraction. Those are also the altitudes where you need to worry about correcting for the flattening of the Sun/Moon (angular dependence for semi-diameter) when clearing the lunar distance.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
